This invention relates to an X-ray computer tomography (CT) apparatus having a changing mechanism of a tilt angle of an X-ray scanner.
JP-A-4-227238 as a laid-open publication of a Japanese patent application discloses a prior art example for correcting an error of an X-ray fan beam position. This prior art example teaches to correct the error of the X-ray fan beam position in a detector mechanism resulting from a mal-arrangement of the position of a focal point spot of an X-ray tube. In the first concrete example, the movement of an irradiation area of the X-ray fan beam resulting from the mal-arrangement of the focal spot of the X-ray tube (movement in a slice width direction) is detected by a Z-axis offset detector (when a coordinates system set in the slice width direction is defined as the Z axis) and position control of a collimator is effected in such a manner as to eliminate this offset. In the second concrete example, mechanical deflection or a thermal drift of the focal spot of the X-ray is calculated by using an anticipation model and position control of the collimator is effected so as to correct the spot position fluctuation of the X-ray tube obtained by this calculation. The error can be eliminated by correcting the fluctuation of the focal spot of the X-ray tube.
The anticipation model described above includes a thermodynamic/geometric model and a mechanical stress model. The former anticipates the fluctuation of the focal spot of the X-ray tube due to the thermal drift and the latter anticipates the fluctuation of the focal spot of the X-ray tube determined or analyzed empirically by a function of a rotating speed and a tilt angle of an X-ray scanner.
The mechanical stress model of the prior art example described above acquires the fluctuation of the focal spot of the X-ray tube determined or analyzed empirically as a function of the rotating speed and the tilt angle of the X-ray scanner, but a problem remains unsolved in practice in that a correct model is difficult to create.
When, for example, any shake (play) exists at a support portion of a rotating anode of the X-ray tube as an X-ray source, this cannot be easily expressed as a formula manipulation model. Further, when tilt angle control (that is, tilt angle for tilting forward or rearward the X-ray scanner) is conducted, there develops often the case where the X-ray fan beam position does not coincide under the erecting state when the X-ray scanner is returned from the forward tilt position to the erecting state and when it is returned from the rearward tilt state to the erecting state. Nonetheless, the formula manipulation model does not take this discrepancy into account. When tilt angle control of the X-ray scanner is effected, influences of the gravity and the centrifugal force may exist but these influences are not taken into account. For these reasons, a ring artifact is likely to develop in a slice image.